1. Field of the Invention
This invention relates to a real image mode variable magnification finder optical system for use in photographic cameras or video cameras and, more specifically, in cameras whose bodies are small in thickness.
2. Description of the Related Art
Reversed Galilean finder optical systems and Keplerian finder optical systems are well known as conventional finder optical systems in which a photographic optical system is provided independent of a finder optical system. The reversed Galilean finder optical system has defects in that the view of a field frame is unclear and the visual field is blurred by ghosts and flares attributable to a half-silvered mirror disposed to form the field frame. Furthermore, where the optical system is constructed as a variable magnification optical system in particular, its overall length and outside diameter are both increased, and a variable magnification ratio cannot be improved. In contrast to this, the Keplerian finder optical system is adapted to observe a real image formed by an objective lens through an eyepiece, and thus overcomes the problem of blurring the visual field for observation caused by the reversed Galilean finder optical system.
Where the Keplerian finder optical system is provided with the function of varying magnification, it is small in size and high in variable magnification ratio compared with the reversed Galilean finder optical system. Hence, the variable magnification finder optical systems have found many applications in the Keplerian finder optical systems. An example of such optical systems is set forth in Japanese Patent Preliminary Publication No. Hei 4-219711. This optical system is such that a variable magnification objective optical system is composed of a first lens unit with a negative refracting power, a second lens unit with a positive refracting power, and a third lens unit with a positive refracting power, and a first reflecting surface for erecting an image is disposed between the third lens unit and an intermediate imaging plane. Hence, the overall length of the finder in a direction along the optical axis of incidence on the finder optical system (which is hereinafter referred to as a longitudinal length) is practically determined by the length of the variable magnification objective lens. This gives rise to the drawback that the thickness of the camera body cannot be made smaller than the length of the variable magnification objective lens. Further, in order to reduce the longitudinal length in the arrangement mentioned above, it is necessary to increase the refracting powers of individual lens units and reduce the length of the objective lens. The increase of the refracting powers of individual lens units, however, will cause the problems that distortion and other aberrations are deteriorated, the influence of errors on the optical system is increased, and lenses are difficult to make.
Thus, the methods of compactly incorporating a finder optical system in the camera body to decrease the thickness of the camera body are known from optical systems disclosed by Japanese Utility Model Publication No. Sho 33-7067 and Japanese Patent Preliminary Publication No. Hei 3-287216.
In the former optical system, however, a prism having the first reflecting surface for erecting an image is disposed in front of a variable magnification objective lens. For this reason, where the exit pupil is provided in the finder optical system, the prism must be enlarged in order to ensure a beam of light required for the finder optical system. Moreover, since a path-splitting prism for a rangefinder is situated ahead of the above prism, the finder optical system must be disposed adjacent to the backside of the camera body. This makes it impossible to reduce the longitudinal length of the finder optical system and attain the smallest possible thickness of the camera body. Further, besides the fact that the second reflecting surface for erecting the image is situated behind an intermediate imaging plane, the above prism is made large-sized, and hence it is also impossible to reduce the entire length of the finder optical system in a direction perpendicular to the optical axis of incidence thereon (which is hereinafter referred to as a lateral length).
In the latter optical system, on the other hand, the first reflecting surface for erecting the image is situated between two moving lens units, and thus it is necessary to separately move the first and second lens units in directions at right angles to each other. Consequently, the optical system has the problem that a mechanism for moving lenses becomes complicated.